1. Field of the Invention
The present invention relates to an antenna device available for small communication devices, such as for example a wearable apparatus, a mobile telephone and PHS, which are used in the vicinity of a human body, to a mobile terminal equipped with the antenna device, and to an RFID tag.
2. Related Art
Conventionally, there is a problem that radiation characteristics of an antenna are degraded in the case where a lossy dielectric such as a human body is present in the vicinity of the antenna. In order to solve this problem, it is considered to reduce absorption of a radio wave by the human body using an antenna having unidirectional directivity in the direction opposite to the human body. As means effective to provide an antenna with the unidirectional directivity, there is a method of attaching a ground plane to the antenna. A dipole antenna fitted with the ground plane is a typical example of the method. However, in this constitution, the antenna itself becomes comparatively large, resulting in a problem that it is difficult to mount the antenna to a small terminal apparatus and the like.
Further, power is generally fed to a radio apparatus in an unbalanced state (for example by a coaxial feeder, a microstrip line and the like), but the dipole antenna is a balanced type element, and hence, a balance-unbalance converter (balun) is needed at a feed point. This causes the radio apparatus to be complicated and makes the miniaturization of the apparatus difficult.